(1) Field of the Invention
The present invention relates to a method for manufacturing a field emission display (below FED) and, more specifically a method for manufacturing a field emission display which can obtain good light emission characteristics by forming cathodes simply and in the uniform height.
(2) Description of the Prior Art
A field emission display FED is a kind of flat display provided with tip-type or wedge-type, cathodes and anodes with a layered phosphor. An electron emitted from a certain cathode strikes the phosphor, so that the phosphor is excited to emit the light thereby displaying patterns, characters or signs. Also, despite minimum voltage consumption, color patterns with high resolution and brightness can be displayed.
A conventional FED of microtip-type disclosed in U.S. Pat. No. 4,908,539 and JP unexamined Publication No. Sho 61-221783 will be described in connection with FIG. 3.
Gates 3 of the rows of electrodes which are divided by cathode patterns 2 and insulating coatings 4 and have a plurality of holes 30 are disposed on a back glass substrate 1 in the cross shape. A plurality of cells 5 are formed on the cross parts. In the cell 5, the same number of microtips 6 as that of holes 30 are formed on the cathode pattern 2. Spacer 7 covering each cell 5 is disposed on the top side of the cell 5. In the meantime, an Indium Tin oxide ITO transparent conductive coating 9 forming an anode electrode and a phosphor coating 10 are formed on the bottom side of a front glass substrate 8.
FIG. 4 describes in an enlarged sectional view the above FED cell 5. As shown in this figure, the microtip 6 is a cathode of a cool cathode using a high electric field emission. Its end is pointed as a tip-type. Even though a lower voltage is applied to the tiny area, electrons are emitted from the end of the tip-type cathode thereby exciting the phosphor 10 facing the cathode.
Namely, electron emission is solicited from a plurality of microtips 6 formed on the cathode pattern 2 and electrons therefrom strike the phosphor 10 through the gates 3 converging the electric field. So, the phosphor 10 is stimulated so as for electrons to be excited. Using the light generated therefrom, the needed picture display can be performed.
In the meantime, the above FED microtip is formed by a process comprising steps shown in FIGS. 5A to 5F.
As shown in FIG. 5A, on the back glass substrate 1, the cathode pattern 2, the insulating coating 4 and the gate 3 are successively formed. As shown in FIG. 5B, a certain portion of the gate 3 is etched by a dry etching to form a hole of the diameter of about 1.4 .mu.m. As shown in FIG. 5C, the insulating coating 4 is etched by a silica etching to form a cavity 40 under the hole 30. As shown in FIG. 5D, with the rotation of the back glass substrate 1, the electron beams are deposited in the projecting angle of 5.degree.-25.degree. to form a nickel layer 11. As shown in FIG. 5E, as well as FIG. 5D, with the rotation of the back glass substrate 1, Mo is deposited on the inner surface of the cavity 40 of the insulating coating 4 to form the microtip 6. After that, as shown in FIG. 5F, Mo deposition 12 with the Ni layer 11 formed on the top of the gate 3 is removed.
Also, a spacer 7 is formed on the whole area of the gate 3 of the back glass substrate 1 except the cell part 5. On the top side of the spacer 7, the front glass 8 on which the transparent conductive film 9 and the phosphor coating 10 are formed are disposed, thereby completing the FED.
However, the microtip 6 formed therefrom, can be easily damaged due to an ion bombardment that, when the electron emitted from the tip excites the phosphor, the positive ion abrades the cathode. As a result, according to the abrasion, the efficiency of electron emission becomes reduced so as not to maintain the stable picture quality thereby shortening the useful life.
Also, when depositing the Ni layer 11 on the gate 3, because the projecting angle of a depositing device (not shown) is modulated with rotating the glass substrate 1, the projecting angle of the depositing device is changed according to the position on the substrate, resulting in the non-uniform tip shapes.
Accordingly, the electron emission force formed on the tip portion becomes non-uniform resulting in the non-uniform brightness. Also, this method has difficulties in forming a plurality of tips at the appropriate uniform height due to the necessary high technology during manufacturing process as well as performing the complicated process.
The above problems act as a big defect when manufacturing a large FED. The combining force of the cathode tip exciting the electron emission with the cathode electrode is weak because, during the manufacturing process of the FED, in each etching step, the etchant is penetrated into the contacting portion of the cathode tip and the cathode electrode, so that, at the time of driving, the cathode tip is dropped out resulting in the reduced manufacturing efficiency.